I would say light as well, but you have loads of light above the tank. Is the plant in any major shadow at all?

I woudlnt say that your photoperiod is too short.. lanky growth is more a sign of insufficient light intensity, rather than anything else.

If you have no shadows, maybe your tubes need cleaning, PC tubes do get pretty mucky after a while.. might be that unless theyre really really old ones, but even then, you have so many of them I doubt that would be it.